1. Field of the Invention
This invention relates to acoustic attenuation and vibration damping materials, particularly to acoustic attenuation and vibration damping materials intended to be placed between acoustic and/or vibratory energy sources and acoustic and/or vibration protected areas.
2. Description of Related Art
Numerous methods currently exist for the control of acoustic noise and vibration ranging from simple, passive barrier and damping techniques to more sophisticated electronic noise canceling approaches. These methods may target either the noise or vibration source, the transmission path, the receiving site, or any or all of the preceding in combination. The instant invention is of the barrier class and utilizes a composite material composed of a matrix material containing filler particles with high and/or low characteristic acoustic impedances to provide improved sound attenuation, vibration damping, and weight characteristics. By careful selection of matrix material and filler particles, the design engineer can create a composite material with an optimal balance of sound and vibration attenuation, weight, strength, temperature characteristics, and durometer.
Within the field of noise control, absorptive techniques are typically utilized to prevent or reduce air-borne acoustic energy from reaching a receiving site. Similarly, vibration damping techniques are usually applied in close contact with the vibrating structure to prevent or reduce air-borne or structure-borne energy from propagating to the protected area. Both techniques utilize internal damping of impinging acoustic energy as an important means of reducing energy levels and therefore share basic principles. A general review of the art in this area is available from "Material Damping and Slip Damping" by L. E. Goodman (Shock & Vibration Handbook (3rd ed.), Cyril M. Harris (ed.), 1987) and from "Sound-Absorptive Materials" by Ron Moulder (Handbook of Acoustical Measurements and Noise Control (3rd ed.), Cyril M. Harris (ed.), 1991), but a brief overview follows.
Currently available materials capable of absorbing unwanted acoustic energy (i.e., noise) are most effective at frequencies above 500 Hz. Noise attenuation rapidly worsens as lower frequencies are encountered with the result being that few material manufacturers even report attenuation values below 125 Hz.
Most sound absorptive materials, such as foams, felts, etc., are highly porous in structure with the pores intercommunicating throughout the material. The pores may be formed by interconnected solid bubbles, or interstices between small granules, or they may be inherent in naturally porous fibrous materials such as fiberglass. The amplitude of sound waves entering the porous material is reduced through friction between the air molecules and the surfaces of the pores. These materials tend to be light in weight and most effective at shorter wavelengths (i.e., higher frequencies). Unless these porous materials form part of a layered, or constrained, composite with a denser, less porous material, their structural strength is limited.
In order to attenuate lower frequencies, absorptive materials are usually combined with a rigid material with an air space separating the two materials. The amount of low frequency attenuation is directly related to the size of this air space. This approach of combining a sound absorptive material with a rigid material and a separating air space increases both the overall weight and thickness of the resulting sound attenuating structure and therefore may not be feasible in a given application. A significant problem with this approach is the fact that many structures must be load bearing as well as sound absorbing, necessitating the inclusion of solid members between rigid materials. These solid members often provide a very good conduit for acoustic energy, thereby partially defeating the structures' sound attenuating properties.
Another approach embodies the "mass law" which applies to a relatively thin, homogeneous, single layer panel. The mass law states that the loss of energy as it transits a barrier is, over a wide frequency range, a function of the surface density of the barrier material and the frequency in question. In general, this transmission loss increases by 6 dB for each octave increase in frequency and for each doubling of the mass of the material. Thus, increasing the mass of the material through increases in thickness or density can improve the acoustic barrier for all frequencies including those in the lower portion of the spectrum. This gain in transmission loss is at the cost of added barrier weight.
Materials utilized specifically for vibration damping follow many of the same rules as those in the absorptive class but are, as a general rule, optimized for attenuating the lower frequencies. As a result, many of these materials are of higher densities and thicknesses and tend to depend more on the internal damping of energy penetrating the material than upon the "capture" of acoustic energy by way of a porous architecture.